docs: add christian-schab as a contributor for plugin

[allcontributors]

Adds @christian-schab as a contributor for plugin.

This was requested by abuaboud in this comment

[skip ci]

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}